1. Field of the Invention
The present invention relates to a plastic extruder assembly and more specifically to a die assembly having means for keeping the die head of the assembly insulated from an outer die face to inhibit freezing of the molten plastic as it is extruded through the die assembly.
2. Description of the Prior Art
Extruders for extruding plastic material into a molten stream of plastic material have been known and used for some time. A particular use of such an extruder is in conjunction with a pelletizer which is mounted to one end of the extruder. By way of example, the extruder may include an inlet feed hopper positioned above a barrel of the extruder assembly. A feed screw is positioned in the extruder assembly, and rams are mounted for movement in the hopper for the purpose of pushing lumps of molten plastic material (such as batches of pigmented polyethylene) into engagement with the feed screw for movement of the plastic material toward the forward end of the barrel of the extruder assembly. The plastic then passes through a die head in a die assembly and the plurality of streams of plastic exiting from the die assembly are typically cut into pellets by a pelletizer.
One particular use of an extruder of the type described is in connection with a pelletizer which may be mounted to the end of the extruder. The preferred embodiment of the present invention will be described with reference to such a specific extruder and pelletizer apparatus, however, it is to be understood that the novel features of the present invention can be adapted to the die assembly of any extruder, used with or without a pelletizer. The extruder of the preferred embodiment includes a die head having a plurality of holes therein. The die head is mounted at the end of the extruder with an outer die face plate. The pelletizer used in conjunction with the extruder, is disposed at the exit of the extruder such that the die assembly also forms part of the entrance to the pelletizer assembly. The pelletizer includes a rotating cutter having cutting blades positioned adjacent the die face of the face plate from which streams of molten plastic material flow. The rotating cutter assembly cuts the streams of plastic material into pellets of various sizes depending upon tnhe extrusion flow rate through the holes in the die head and the speed of rotation of the cutter assembly In some pelletizers, the flow of water in the chamber housing the cutter assembly serves to carry the pellets into a receiving container.
In extruder devices where the plastic streams are formed by passing molten plastic through holes in a die head, the holes tend to clog when some of the molten plastic freezes or hardens so as to decrease the diameter of the die hole. Such freezing causing reduction of the diameter of the die hole obviously impedes the flow of plastic resulting in "off-spec" pellets and if one or more die holes are frozen shut, the number of streams of plastic being produced at a given time is reduced. Both phenomenon substantially reduce the efficiency of the apparatus.
Accordingly, it is desirable to provide a die assembly for an extruder of the type described wherein build up in or complete clogging of die holes is reduced if not obviated, whenever clogged die holes may be readily unclogged, and wherein die hole diameters can be changed to vary the size of the plastic stream produced by the extruder. In most present die assemblies, it is necessary to remove the entire die head to unclog die holes.
Also, it is desirable to provide means for easily changing the size of the die holes since in present die assemblies it is necessary to remove and replace an entire die plate assembly if die holes of a different size are desired.
In addition, it is desirable to provide an efficient system for heating the plastic just prior to its being passed through the holes in the die assembly, and for ensuring that the temperature of the die assembly is maintained at an elevated state as the extruding process proceeds.
Also, it is desirable to construct a die assembly having the die holes therein disposed in such an array that the entire length of the cutter blade is sequentially used in cutting the plastic streams into pellets, rather than having only one small section of each blade come into contact with the plastic. This prevents excessive wear at one location on the blades, and vastly increases the useable life of the blades.
As will be described in greater detail hereinafter, the extruder apparatus of the present invention provides an insulating device between a die face plate having an outer die surface adjacent which the cutters rotate and a die head or body having die holes therein to prevent freezing of the molten plastic in the die holes.
In addition, the present invention provides a die assembly for an extruder of the type described which allows clogged die holes to be readily opened, and also allows the diameter of die holes to be easily changed to modify the size of the plastic streams produced by the extruder assembly.
Further, the present invention provides a hot oil passageway in the die assembly for maintaining the temperature of the die body at an elevated level to inhibit freezing of the plastic in the die holes, and which works in association with the aforesaid insulation device to keep the die body of the die assembly at an elevated temperature despite the possible presence of a pelletizer cooling system, or a cooler ambient temperature at the output side of the die assembly. To this end, the die assembly of the present invention is provided with a unique configuration of passageways which enhance the flow of hot oil adjacent and/or around the die holes to increase the transfer of heat to the holes through which the molten plastic is passing. Additionally, a hot oil passageway is provided in a die spider for heating the molten plastic just before it enters the die assembly.
Still further, the present invention provides a unique configuration of die holes in a die plate assembly whereby a cutter blade passing over the die plate will contact and cut the plastic streams at various points along the length of the blades to provide uniform wear of the blades.